In order to measure a flow rate of a fluid to be measured flowing through a flow tube, a vortex flowmeter and a thermal flowmeter are used.
As is generally known, the vortex flowmeter makes use of the fact that the number of Karman vortexes (vortex frequency) generated by a vortex generator in a unit time is proportional to a flow rate within a predetermined Reynolds number range regardless of whether the vortex generator is a gas or a liquid when the vortex generator is placed in a fluid flow. The constant of proportionality is referred to as the Strouhal number. As vortex detectors, a thermal sensor, a strain sensor, an optical sensor, a pressure sensor, an ultrasonic sensor and the like are given. The above vortex detectors can detect a thermal change, a change in lift or the like caused by the vortexes. The vortex flowmeter is a simple flowmeter capable of measuring a flow rate without being affected by physical properties of the fluid to be measured, and is widely used for flow rate measurement of gases or fluids (for example, see Japanese Patent No. 2869054).
The thermal flowmeter includes a temperature sensor (fluid temperature detecting sensor) and a heating temperature sensor (heating-side temperature sensor). A temperature of the heating temperature sensor (flow velocity sensor (heater)) having a function of a temperature sensor and a function of a heating sensor is controlled to have a constant difference in temperature with respect to a temperature measured by the temperature sensor. This is because a heat quantity, which is removed from a heater when the fluid to be measured is caused to flow, is correlated with a mass flow rate. The mass flow rate is calculated from electric power for heating the heater (for example, see Japanese Patent Application Laid-open No. 2004-12220).
Japanese Patent Application Laid-open No. 2006-29966 discloses a technology of a multi-vortex flowmeter including both the function of the vortex flowmeter and the function of the thermal flowmeter. The multi-vortex flowmeter is capable of measuring a flow rate ranging from an extremely low flow rate to a high flow rate with good accuracy, and is particularly superior to the other flowmeters in this point.
The multi-vortex flowmeter can selectively use the function of the vortex flowmeter and that of the thermal flowmeter according to the condition of a flow of the fluid to be measured flowing through a flow channel of a flow tube. Specifically, the function of the thermal flowmeter is used to perform a measurement in an extremely low flow rate region and a low flow rate region, whereas the function of the vortex flowmeter is used to perform a measurement in a high flow rate region.
Since a sensitivity of the vortex detector is insufficient in the vortex flowmeter when the flow rate is lowered to reduce a vortex differential pressure, control is performed to switch the function to that of the thermal flowmeter at a predetermined lower limit flow rate in the multi-vortex flowmeter.